This invention relates to a thermographic strip chart recorder. It relates more particularly to a variable temperature stylus for use in recorders of this type.
In thermographic recorders, a strip of chart paper is advanced past a heated recording stylus. Simultaneously, the stylus is moved in a direction generally perpendicular to the strip advance by means of a pen motor in accordance with output signals from a sensing transducer of some kind. The chart paper is chemically treated so that its color changes where it is engaged by the heated stylus. Resultantly, a visible track is formed on the paper strip which reflects the instantaneous value of the condition being sensed.
For example, the recorder may receive the output of a pressure transducer connected to a patient's wrist and thus indicate changes in the patient's pulse rate with time. As another example, the recorder may receive the output of a sonar transponder so that the graph on the strip reflects changes in ocean depth along a particular track followed by a ship, the advance of the strip corresponding to the ship's advance along the track.
Most recorders include provision for selecting different chart speeds for scaling purposes. It is a characteristic of thermographic recorders that the discoloration of the chemically treated paper by the heated stylus varies with chart speed. That is, if the paper strip advances relatively slowly, the heated stylus spends more time at a given location on the paper and therefore the paper is heated to a greater extent resulting in a relatively dark trace at that location. Conversely, as the paper speed increases, it is heated to a lesser extent by the stylus at each location and is therefore lighter and less visible. Of course, it is desirable for the chart track to be uniformly dark at all chart speeds. Accordingly invariably provision is also made for varying the temperature of the stylus depending upon the speed of the advancing chart paper so that the trace on the chart is clearly visible at all times.
Conventional recorders of this general type are usually powered by a D.C. power supply that produces, typically, 12 volts and its stylus requires about 6 watts maximum power, e.g. 2 volts at 3 amperes, to provide a distinct trace. Current to the stylus is often applied by way of a variable power resistor or potentiometer which can be adjusted to vary the power applied to the stylus from a maximum of of 6 watts required at maximum chart speed through one or more intermediate settings to a minimum of about 2 watts required at the slowest chart speed. This means that at chart speeds other than maximum speed, part of the power developed by the D.C. supply is dissipated as heat in the potentiometer and is, therefore, wasted. Furthermore, the temperature of the stylus cannot be controlled as precisely as one might want using variable resistance controls of that type.